1. Field of the Invention
The present invention relates to x-ray phosphor powders, methods for producing x-ray phosphor powders and devices such as x-ray image intensifiers incorporating the powders. In particular, the present invention is directed to x-ray phosphor powders having well controlled chemical and physical properties. The present invention also relates to a method for producing such powders by spray-conversion.
2. Description of Related Art
Phosphors are compounds that are capable of emitting useful quantities of radiation in the visible and/or ultraviolet spectrums upon excitation of the material by an external energy source. Due to this property, phosphor compounds have long been utilized in cathode ray tube (CRT) screens for televisions and similar devices. Typically, inorganic phosphor compounds include a host material doped with a small amount of an activator ion. X-ray phosphor powders are used in image intensifiers, particularly for medical devices.
There are a number of requirements for phosphor powders, which can vary dependent upon the specific application of the powder. Generally, phosphor powders should have one or more of the following properties: high purity; high crystallinity; small particle size; narrow particle size distribution; spherical morphology; controlled surface chemistry; homogenous distribution of the activator ion; good dispersibility; and low porosity. The proper combination of the foregoing properties will result in a phosphor powder with high luminescent intensity and long lifetime. It is also advantageous for many applications to provide phosphor powders that are surface passivated or coated, such as with a thin, uniform dielectric or semiconducting coating.
Numerous methods have been proposed for producing phosphor particles. One such method is referred to as the solid-state method. In this process, the phosphor precursor materials are mixed in the solid state and are heated so that the precursors react and form a powder of the phosphor material. For example, U.S. Pat. No. 4,925,703 by Kasenga et al. discloses a method for the production of a manganese activated zinc silicate phosphor (ZnSiO.sub.4 :Mn). The method includes a step of dry blending a mixture of starting components such as zinc oxide, silicic acid and manganese carbonate and firing the blended mixture at about 1250.degree. C. The resulting phosphor is broken up or crushed into smaller particles. Solid-state routes, and many other production methods, utilize such a grinding step to reduce the particle size of the powders. The mechanical grinding damages the surface of the phosphor, forming dead layers which inhibit the brightness of the phosphor powders.
Phosphor powders have also been made by liquid precipitation. In these methods, a solution which includes phosphor particle precursors is chemically treated to precipitate phosphor particles or phosphor particle precursors. These particles are typically calcined at an elevated temperature to produce the phosphor compound. The particles must often be further crushed, as is the case with solid-state methods.
In yet another method, phosphor particle precursors or phosphor particles are dispersed in a solution which is then spray dried to evaporate the liquid. The phosphor particles are thereafter sintered in the solid state at an elevated temperature to crystallize the powder and form a phosphor. For example, U.S. Pat. No. 4,948,527 by Ritsko et al. discloses a process for producing Y.sub.2 O.sub.3 :Eu phosphors by dispersing yttrium oxide in a europium citrate solution to form a slurry which is then spray dried. The spray dried powder was then converted to an oxide by firing at about 1000.degree. C. for two hours and then at 1600.degree. C. for about four hours. The fired powder was then lightly crushed and cleaned to recover useful phosphor particles.
U.S. Pat. No. 5,644,193 by Matsuda et al. discloses phosphor powders having an average particle size of up to 20 .mu.m. The phosphors can include rare earth oxides, rare earth oxysulfides and tungstates. The particles are produced by fusing phosphor particles in a thermal plasma and rapidly cooling the particles.
Despite the foregoing, there remains a need for x-ray phosphor powders with high luminescent intensity that include particles having a substantially spherical morphology, narrow particle size distribution, a high degree of crystallinity and good homogeneity. The powder should have good dispersibility and the ability to be fabricated into thin layers having uniform thickness. Phosphor powders having these properties will be particularly useful in x-ray image intensifiers.